1. Field of the Invention
The present invention relates to an agricultural tractor with a suspended drive axle and in particular to a tractor having an independent rear suspension.
2. Description of the Related Art
An agricultural tractor is intended primarily for off-road usage and is designed primarily to supply power to agricultural implements. An agricultural tractor propels itself and provides a draft force in the direction of travel to enable an attached, soil engaging, implement to perform its intended function. Furthermore, an agricultural tractor may provide mechanical, hydraulic and/or electrical power to the implement. Agricultural tractors must be designed with sufficient normal force, down force, acting on the drive wheels to produce the needed draft force. Typically, in a two-wheel drive tractor, this results in a vehicle having rear drive wheels that are larger then the front wheels to accommodate the required normal force on the rear drive wheels. Agricultural tractors differ from cargo carrying vehicles, such as pickup trucks and semi-truck tractors, in that trucks do not need to produce a continuous draft load. A truck produces a draft load only during periods of acceleration and deceleration and relies on the weight of the cargo carried by the drive wheels to produce the draft load.
The productivity of an agricultural tractor can be increased by faster travel speeds in the field and on the road. A significant limiting factor to the travel speed of an agricultural tractor is the comfort of the operator when travelling over rough surfaces. A typical tractor has an integrated structure in which the rear axle and hitch are integrated into one unit and forms a part of the vehicle frame structure. In such a case, there is no suspension between the rear axle and the tractor frame. Tire deflection alone provides a cushioning between a rough surface and the tractor frame. Seat suspensions and cab suspensions have also been used with limited success to improve operator comfort.
The front axle, which may or not be driven, is typically a beam axle pivotally mounted to the tractor for limited rotation about a longitudinal axis. Suspended front axles have been developed for tractors such as that shown in U.S. Pat. No. 5,879,016. There, a rigid beam axle having outboard planetary final drives is suspended from the tractor frame. Suspended front axles have provided improved comfort for operators.
However, due to the lack of a rear suspension, significant loads caused by an uneven terrain are still transmitted to the vehicle frame and to the vehicle operator. Tractor speed, particularly in the field, is limited by the jostling of the operator. Operator fatigue occurs sooner the more the operator is jostled in the cab. Thus there exists a need for a rear suspension in an agricultural tractor to reduce operator fatigue, thereby allowing the operator to productively work longer hours and/or to operate at a faster travel speed.
One attempt at providing a rear suspension is shown in U.S. Pat. No. 5,538,264. There, a rear beam axle is suspended from the tractor frame. To include the suspension, the tractor design deviates from a conventional row crop tractor of comparable power in at least the following respects: 1) the rear axle is equipped with outboard final planetary drives instead of inboard final drives; 2) the tractor is equipped with four equal sized tires, all smaller than the large rear tires on a conventional row crop tractor of comparable power; and 3) the rear axle is moved substantially rearward relative to the tractor cab, as compared to a conventional row crop tractor.
The outboard final drives limit the adjustability of the tread width and prevent the tread width from being infinitely adjustable along the axle. Only discrete changes in the tread width are available through different wheel and rim configurations. Of the commercially available tractors embodying the invention of the U.S. Pat. No. 5,538,264, only to lowest power tractors are available with a tread width as narrow as 60 inches. The smaller tires have a lower load carrying capability, resulting in less traction. The rearward location of the rear axle relative to the cab interferes with visibility of the hitch by the operator. An additional disadvantage of this design is that the tractor hitch is carried by the suspended axle, and is thus part of the unsprung mass. The lack of a suspension for the hitch results in towed implements following the vertical motion of the tires and axle as opposed to the more steady movements of the tractor frame. A further disadvantage is that the beam axle does not enable an independent suspension.
As a result of all the differences between the tractor of the '264 patent and a conventional row crop tractor, the '264 tractor does not achieve the same level of performance of a comparable power row crop tractor. The tractor of the '264 patent does allow increased travel speeds, both in the field and on the road, but at a significant ‘cost’ in terms of performance in the field. The tractor of the '264 patent is designed more for use as a hauling vehicle and for road transport than for field work pulling a soil engaging implement. Accordingly, there still exists a need for a row crop tractor having a rear suspension while maintaining the performance and operational characteristics of a row crop tractor of comparable power with a rigid, non-suspended, rear drive axle.
A row crop agricultural tractor is designed to operate in the field with the tread width set for the wheels to travel between rows. Conventionally, this has meant a tractor with a tread width as narrow as 60 inches, enabling the tractor to straddle two crop rows spaced 30 inches apart. The tread width is adjustable so that it can be customized for a particular farm application with row spacings other than 30 inches. Infinite adjustability can be provided by wheel hubs that clamp to an axle at any location along a length of the axle. This type of adjustment mechanism necessitates an inboard planetary final drive. Other adjustment mechanisms involve changing the wheel rim and disc configuration to change the tread width.
Small row crop tractors such as the JOHN DEERE 6010 Series tractors have power ratings ranging between 49 kW to 71 kW (65-95 hp). Large row crop tractors such as the JOHN DEERE 8010 series tractors range in power from 123 kW to 175 kW (165 to 235 hp). These tractors all have inboard final drives to enable an infinitely adjustable tread width. In the JOHN DEERE 8010 series tractors, the width of the rear differential case, between the inboard planetary final drives, is 665 mm (26.2 inches). A rear suspension must be packaged outboard of the final drive and inboard of the wheel mounting equipment at a 60 inch tread width to maintain the same row-crop capabilities in a suspended tractor of this size as in the non-suspended tractor.